There is known from Japanese Patent Laid-open No. Hei 2-150517 a so-called assistive slipper clutch having a cam mechanism for pulling in a clutch cam member and increasing the urging force of a clutch spring to increase the pressing forces of drive friction plates and driven friction plates upon acceleration in a power transmitting state and pushing out the clutch cam member and reducing the urging force of the clutch spring to reduce the pressing forces of the drive friction plates and the driven friction plates upon deceleration.